The present invention relates to a process where a liquefied or dissolved substance is crystallized from a melt or a solution while exposing it to ultrasound. A triglyceride fat (three fatty acid residues connected to a glycerol backbone) in particular is the subject of the present crystallisation process.
The triglyceride fats used for the manufacture of food compositions often are desired to show a specific melting behaviour. Fats as obtained from natural sources usually do not have suitable melting properties. Therefore they have to be subjected to a modification treatment. Fat fractionation is such a modification treatment. Fat fractionation consists of the physical separation of a triglyceride mixture into two or more fractions with different melting or solubility ranges. xe2x80x9cWetxe2x80x9d fractionation comprises dissolving the triglyceride mixture in a hot organic solvent (e.g. hexane) and then cooling it slowly until a part (fraction) of the fat crystallizes from the solution.
Alternatively, xe2x80x9cdryxe2x80x9d fractionation does not make use of a solvent and comprises cooling a liquid fat slowly. Optionally a triglyceride mixture is first fully liquefied if it is solid. The fat fraction with the highest melting range will crystallize first during cooling.
The final stage of both wet and dry fractionation is separation of the crystallized (xe2x80x9cstearinxe2x80x9d) fraction and the still liquid (xe2x80x9coleinxe2x80x9d) fraction by filtration.
Dry fractionation is the preferred option when a xe2x80x9cnon-chemicalxe2x80x9d modification treatment is desired. For dairy fats it is the only acceptable option in terms of retaining flavour quality. However, dry fractionation is a less efficient and controllable method than wet fractionation (Ref.1).
The filter cake resulting from wet fractionation may contain as little as 2 wt. % entrapped liquid fraction (also denoted as 98% SE (separation efficiency)). The good result is due to a more favourable crystal morphology and to washing the crystallized fraction with clean solvent. By contrast, the solids content in the cake resulting from a standard dry fractionation process typically is at most about 60% (60% SE), the remaining 40% being entrapped olein.
Crystal habit modifiers (CHM""s) when added to the melt modify the crystal morphology such that more compact crystals may be produced which can be better separated from the liquid olein phase. The use of CHM""s may increase the SE to about 80%, but at the expense of a much increased process time. CHM""s slow down both nucleation and crystal growth. Moreover, for the removal of the CHM""s from the desired fat fractions additional post-processing is necessary.
Sonocrystallisation is the use of ultrasound for influencing the crystallisation of liquids, either melts or solutions. Ultrasound in common language is sound characterized by a frequency of about 20 kHz and more, extending even into the MHz range. Most applications use ultrasound in the range 20 kHz-5 MHz.
The  greater than 20 kHz frequency for defining ultrasound is rather arbitrary and is historically related to the average perception limit of the human ear. Within the context of the present specification such perception limit is irrelevant from a technical point of view. The benefits of the present invention become manifest as well with frequencies well below 20 kHz. In the context of the present specification ultrasound is defined as sound with a frequency of 10 kHz up to 10 MHz.
Since 1927 it is known that by exposing supercooled melts or supersaturated solutions of various substances to ultrasound the nucleation and/or the growth of crystals is remarkably influenced. The effect, sonocrystallisation, was first observed when crystallizing a supersaturated thiosulfate solution. Since then sonocrystallisation has been studied in many other systems. A particular aspect of sonocrystallisation is sononucleation. It deals with the initiation of crystal formation, has been studied extensively with sugar and is applied since the late 50-ties. Sonocrystallisation of supercooled water, supercooled metal melts and supersaturated solutions of various inorganic materials have received a lot of attention in the 50-ties and 60-ties, particularly in Russia.
The crystallisation process can be divided into two stages: crystal nucleation and crystal growth. In the nucleation stage submicroscopic crystal nuclei are formed which develop into larger crystals during the subsequent growth stage. With homogeneous nucleation the crystals are formed directly from the liquid. Heterogeneous nucleation is nucleation mediated by foreign particles already present in the liquid. Secondary nucleation is nucleation mediated by pre-existing crystals. It is believed that the process of the present invention predominantly affects homogeneous nucleation.
Benefits of sonocrystallisation reported in literature include:
Faster nucleation which is fairly uniform throughout the sonicated volume,
Relatively easy nucleation of materials for which nucleation is difficult otherwise,
Generation of smaller, purer and more uniform crystals.
For literature dealing with sonocrystallisation see the reviews e.g. of Kapustin (Ref.2) and Hem (Ref.3).
When a liquid is exposed to ultrasound, microscopic gas/vapour bubbles are formed which show a dynamic pulsating behaviour. One activity of such ultrasound-induced bubble behaviour is denoted as cavitation. Already at relatively low sound intensities the bubbles do not perish but exhibit stable volume and/or shape oscillations. This type of cavitation is denoted as xe2x80x9cstablexe2x80x9d or xe2x80x9cnon-inertialxe2x80x9d cavitation. When the ultrasound intensity is increased and exceeds a certain limit, the cavitation threshold, the nature of cavitation changes dramatically which results in the bubbles becoming unstable. Within a fraction of a sound cycle they show rapid growth followed by a violent collapse. The collapsing gas bubbles produce very high pressures and temperatures locally in the bubble as well as a high pressure in the liquid layer surrounding the bubble (see also Hem, 1967, supra).
Cavitation which shows this violent bubble behaviour is denoted as xe2x80x9ctransientxe2x80x9d or xe2x80x9cinertialxe2x80x9d cavitation (ref.5). By many ultrasound users the terms xe2x80x9ccavitationxe2x80x9d and xe2x80x9ctransient cavitationxe2x80x9d are used without discrimination.
According to general scientific consensusxe2x80x94which has persisted until now (see e.g. ref. 4 and 8)xe2x80x94the physical mechanism underlying sonocrystallisation and the benefits resulting from it are ascribed to the occurrence of transient cavitation. The prejudice tells that in the absence of transient cavitation the benefits of sonocrystallisation even will not be manifested.
After the 60-ties the scientific attention for sonocrystallisation seems to have decreased. No fundamentally new insights in the believed underlying cavitation mechanism have been reported. However, the technological development and application of ultrasound for the crystallisation of different materials continued.
A few patent applications relate to sonocrystallisation of edible fats. WO 92/20420 describes a method and a device for the control of solidification in liquids. The liquid to be solidified is subjected to inter alia ultrasonic cavitation in order to control the steps of nucleation and/or crystal growth of the solidification process. In conformity with prevailing views the ultrasonic conditions desired for nucleation induction are chosen such that transient cavitation results which implies high intensity ultrasound.
EP 765605 deals with the effect of ultrasonic treatment on fat nucleation. It describes a method for accelerating the polymorphic transformation of edible fat compositions. Such compositions when undercooled by at least 4xc2x0 C. are exposed to ultrasonic energy for a time and at a frequency sufficient to induce nucleation of stable polymorph crystals without exceeding the melting point of those crystals. Typical fats to be treated by this method are butter fat and the fats used in ice cream, chocolate, margarine and yogurt.
EP 765606 describes a method for retarding fat blooming on chocolate and on other confectionery fat compositions comprising cocoa butter. The method comprises undercooling the molten fat by at least 3xc2x0 C. below the melting point of the xcex2-polymorph crystal. By exposing it to an effective amount of ultrasonic energy stable crystals are generated.
In those patents cavitation is presented as the evident cause of the enhanced nucleation and the changed crystal morphology.
Traditional sonocrystallisation, however, has shown also serious drawbacks. Sonocrystallisation may trigger sonochemical reactions some of which are believed to cause production of free radicals. Triglyceride fats, especially unsaturated oils, are very susceptible to oxidation damage caused by decomposition of lipo(hydro)peroxides formed by free readicals. The resulting off-flavour and off-taste has become a decisive factor preventing the wide use of sonocrystallisation for edible unsaturated fats. A small flavour defect in the predominantly saturated chocolate fats as exemplified in the patents above is hardly noticed and even less when incorporated in chocolate products. Skilled fat chemists have persisted in believing that sonocrystallisation of an unsaturated edible fat is impossible without adversely affecting its taste and smell.
We have found that the beneficial effects of fat sonocrystallisation being necessarily related to transient cavitation are based on a prejudice.
The present inventors have found that sonocrystallisation can considerably enhance the nucleation rate of fat crystallisation also when applied in the absence of transient cavitation. Adverse sonochemistry with its flavour spoiling effects does not occur. A major accomplishment was the significant improvement of the separation efficiency of a dry fractionated oxidation sensitive fat without the expected oxidation damage and without adversely affecting the taste and smell of the obtained fat fractions (see example 4).
Generally, the present invention provides a process for the crystallisation of a solid phase from a liquid which liquid is subjected to ultrasound, where the exposure to ultrasound is at such conditions that transient cavitation is absent and for a time and at a frequency sufficient to induce nucleation of stable crystals in the liquid.